Reconfigurable free space wavelength cross connect

ABSTRACT

An optical cross connect, especially a wavelength cross connect, using free-space optics, a diffraction grating, and a micro electromechanical systems (MEMS) array of movable mirrors. A concentrator receives light from widely separated optical fibers and brings the beams together into a more closely spaced linear array. Free-space optics process all the beams. Front-end optics collimate the beams from the fibers and flatten their fields. The diffraction grating spectrally separates each beam into sub-beams. A long-focus lens focuses the sub-beams onto the 2-dimensional MEMS array. A fold mirror reflectively couples two such mirrors, whereby the switched signals propagate back through the same optics and are spectrally recombined onto the fibers. Other embodiments include white-color cross connects, multiple MEMS arrays, and parallel optics. Power dividers or wavelength interleavers can divide signals from the fibers, and multiple cross connects switch different wavelength groups.

RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional applicationSerial No. 60/283,605 filed Apr. 13, 2001 and is a continuation in partof Ser. No. 09/957,312 filed Sep. 20, 2001.

FIELD OF THE INVENTION

[0002] The invention relates generally to multi-wavelength opticalswitches. In particular, the invention relates to optical switches usingmicro electromechanical system (MEMS) switching elements.

BACKGROUND ART

[0003] Modem communications networks, particularly those extending overlong distances, increasingly use silica optical fiber as thetransmission medium. In the originally implemented fiber-based networks,each fiber carries a single optical carrier at a wavelength in one ofthe silica transmission bands that extend across ranges in theneighborhoods of 850, 1310, and 1550 nm. At the transmitting end, alaser emitting at this wavelength or an associated electro opticalmodulator is modulated by an electrical data signal, and the modulatednarrow-band light is input to the fiber. At the receiving end of thefiber, a photodetector receives the modulated light and converts it backto electrical form. While the fiber itself has a transmission bandwidthmeasured in hundreds of terahertz, the data transmission rates arelimited to the speed of the electronics associated with the transmitterand receiver, currently about 10 gigabits per second. It was quicklyrecognized however that the transmission bandwidth of a fiber can begreatly increased by wavelength division multiplexing (WDM). Forexample, W lasers at the transmitting end, where W may be forty or more,output at respective ones of W wavelengths in one of the silicatransmission bands, and their outputs are modulated by respective datasignals.

[0004] The wavelength spacings in the 1550 nm band may be 1 nm or less.All the modulated optical carriers are combined and input to a singletransmission fiber. At the receiving end of the fiber, a wavelengthdispersive element such as a diffraction grating or prism wavelengthdivides the received multi-wavelength optical signal into W respectivespatial paths. A photodetector and associated electronics are associatedwith each of these paths. Thereby, the transmission capacity of thefiber is increased by a factor of W because of the parallel operation ofW sets of electronics.

[0005] Modem communication networks tend to be more complicated than thepoint-to-point system described above. Instead, most public networksinclude multiple nodes at which signals received on one incoming linkcan be selectively switched to different ones of outgoing links. Forelectronic links, conventional electronic switches directly switch theelectronic transmission signals. Fiber links present a more difficultswitching problem.

[0006] In the most straightforward approach, each node interconnectingmultiple fiber links includes an optical receiver which converts thesignals from optical to electrical form, a conventional electronicswitch which switches the electrical data signals, and an opticaltransmitter which converts the switched signals from electrical back tooptical form. In a WDM system, this optical/electrical/optical (O/E/O)conversion must be performed by separate receivers and transmitters foreach of the W wavelengths. This replication of O/E/O components preventsthe economical implementation of WDM for a large number W of wavelengthchannels.

[0007] Another approach implements wavelength switching in anall-optical network. In a version of this approach that may be used withthe invention, the W wavelength components from an incomingmulti-wavelength fiber are wavelength demultiplexed into the differentspatial paths. Optical switching elements then switch thewavelength-separated signals in the desired directions before amultiplexer recombines the optical signals of differing wavelengths ontoa single outgoing fiber. In conventional terminology, all the opticalswitching elements and associated multiplexers and demultiplexers areincorporated into a wavelength cross connect (WXC), which is a specialcase of an enhanced optical cross connect (OXC). Advantageously, all theoptical switching elements can be implemented in a single chip of amicro electromechanical system (MEMS). The MEMS chip includes atwo-dimensional array of tiltable mirrors which may be separatelycontrolled. Solgaard et al. describe the functional configuration ofsuch a MEMS wavelength cross connect in U.S. Pat. No. 6,097,859,incorporated herein by reference in its entirety. Each MEMS mirrorreceives a unique optical signal of a single wavelength from an incomingfiber and can switch it to any of multiple outgoing fibers. The entireswitching array of several hundred mirrors can be fabricated on a chiphaving dimension of less than 1 cm by techniques well developed in thesemiconductor integrated circuit industry.

[0008] However, such a wavelength optical cross connect needs to beinstalled in the field and to retain its calibration under somewhatharsh conditions without the need for frequent routine maintenance. Itspackaging should be relatively compact to allow its installation inexisting switching facilities and in perhaps remote locations. The costand complexity need to be minimized.

[0009] Smith et al. in U.S. patent application Ser. No. 09/957,312,filed Sep. 20, 2001, incorporated herein by reference in its entirety,sketchily discloses a more compact package including condensed physicaloptics such as folding mirrors. A similar disclosure is published asInternational Publication No. WO 02/25358 A2. However, that descriptionis directed more to features other than the optics.

[0010] A wavelength cross connect advantageously is connected to manyoptical fiber transmission links, and the number of WDM wavelengths isalso advantageously large. The design of the wavelength switching systembecomes increasingly difficult for a large number of input/output fibersand a large number of wavelengths. Further, for a large number of fibersand wavelengths, it becomes increasingly difficult to fabricate all therequired MEMS mirrors in a single substrate.

SUMMARY OF THE INVENTION

[0011] A optical cross connect (OXC) is based on free-space optics andan array of micro electromechanical system (MEMS) mirrors forselectively switching optical signals between waveguides, such asoptical fibers. For a wavelength optical cross connect (WXC), the MEMSmirrors may be arranged in a two-dimensional array, preferably withinthe same plane and more preferably within a same substrate. Thetwo-dimensional mirror array extends in a fiber direction and in aperpendicular wavelength direction.

[0012] Transmission fibers are coupled into the free-space opticsthrough a concentrator which couples on a first side to the fibersspaced by distances representative of the diameter of single-modeoptical fiber, for example, at least 125 μm. The concentrator includesoptical waveguides which curve so that the distances between thewaveguide decrease from the first side of the concentrator to the secondside which has an output facet to the free-space optics. At the secondside, the waveguides are arranged in a linear array spaced by a muchsmaller distance, for example, 20 to 50 μm and couple to beams arrangedaround respective parallel axes in a plane. The concentrator waveguidesmay be planar waveguides formed in a substrate, or they may be opticalfiber aligned to curved grooves formed in a substrate and preferablyhaving ends tapered at the input to the free-space optics.

[0013] The free-space optics may be arranged in and about a principaloptical plane. The fibers or other waveguides are preferably arranged inone or more linear arrays extending a small distance perpendicular tothe principal plane, for example, as determined by the concentrator, andinputting parallel beams to the free-space optics. More preferably, bothinput and output waveguides are arranged in a single linear array. Morepreferably also, a folding mirror reflectively couples selected pairs ofmirrors in the MEMS array with one mirror acting as an input mirror andthe other as an output mirror. The tilts of the selected input andoutput mirrors are controlled in pairs to produce the reflectivecoupling. Further, the MEMS array may be inclined, for example, at 45°to the principal plane to allow the folding mirrors to be placedparallel to the principal plane.

[0014] The free-space optics may include a collimating lens system,preferably including a field-flattening element. A wavelength dispersiveelement, such as a diffraction grating, may be placed between thecollimating lens system and the MEMS array to separate wavelengthcomponents of the waveguided signals. A focusing lens is preferablydisposed between the wavelength dispersive element and the MEMS arrayand has a focus point near the MEMS array. If a fold mirror is used, thefocus point is on or near the fold mirror. Alternatively, the focuspoint may be on the MEMS array or between it and the fold mirror.Closely spaced waveguides ends are imaged onto the MEMS array with amagnification determined by the ratio of the focal lengths of thecollimating lens system and the focusing lens. The magnification ispreferably between 10 and 100, and more preferably between 20 and 50.The optics preferably are designed to be telecentric producing parallelbeams and sub-beams, at least in the wavelength direction.

[0015] The free-space optics may include a prism to compensate for theastigmatism of the wavelength dispersive element, such as a diffractiongrating. By the use of multiple mirrors, the free-space optics may bearranged along a twisted optical axis that crosses itself, therebyreducing the size of the package.

[0016] The scaling of the invention to larger number of fibers or ofwavelengths can be eased by use of multiple MEMS arrays. Multiple MEMSarrays can be bonded onto a common substrate, thus forming a mosaicarray. Separate MEMS arrays can be used for input and output mirrors,and the input and output mirrors of the two arrays may be directlycoupled without the need for a fold mirror. Separate sets of optics andwaveguide arrays may be used on the input and output with the twoarrays. Alternatively, the wavelength separated sub-beams produced bythe wavelength dispersive element may be grouped and directed throughdifferent sets of back-end optics and MEMS arrays.

[0017] The scaling can also be facilitated by splitting the waveguidesignals into two or more parallel paths by means of one or more stagesof divider/combiners, such as power splitters or wavelengthinterleavers. Free-space optical or wavelength interconnects opticallyswitch different wavelengths while assuring that the unswitchedwavelengths are absorbed or otherwise do not interfere.

[0018] The invention also includes the method of operating such opticalcross connects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic illustration of the functions of awavelength cross connect implemented with a mirror array.

[0020]FIG. 2 is an orthographic view of a practical wavelength crossconnect including a micromirror array.

[0021]FIG. 3 is a plan view of a micromirror fabricated as a microelectromechanical system (MEMS).

[0022]FIG. 4 is a cross-sectional view of the micromirror of FIG. 3taken along view line 44.

[0023]FIG. 5 is a fragmentary plan view of a micromirror array andincident beams.

[0024]FIG. 6 is full plan view of the micromirror array of FIG. 5.

[0025]FIG. 7 is a schematic illustration of input and output mirrorscoupled through a folding mirror.

[0026]FIG. 8 is a plan view of an optical concentrator using planarwaveguide.

[0027]FIG. 9 is a cross-sectional view of a first step of a process forforming planar waveguides.

[0028]FIG. 10 is a cross-sectional view of the planar waveguidestructure resulting from the process of FIG. 9.

[0029]FIG. 11 is a cross-sectional view of an optical concentrator usingonly optical fiber.

[0030]FIG. 12 is a schematic view of the input (collimating) optics ofthe wavelength cross connect of FIG. 2.

[0031]FIG. 13 is a plan view of a micromirror array formed as a mosaicof arrays.

[0032]FIG. 14 is a side view of a wavelength cross connect havingseparate input and output optics and mirror arrays.

[0033]FIG. 15 is a schematic illustration of a split beam wavelengthcross connect.

[0034]FIG. 16 is a circuit diagram of a wavelength cross connect inwhich beams are passively split and combined through 3 dB couplers.

[0035]FIG. 17 is a plan view a mosaic of mirror arrays.

[0036]FIG. 18 is a circuit diagram derived from the circuit of FIG. 16using two stages of 3 dB couplers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The invention will be explained in the context of a wavelengthcross connect (WXC) for a wavelength division multiplexing (WDM)communications network using a micro electromechanical system (MEMS)switching fabric. The functions for a WXC of themselves are complex, butwhen implemented in a compact system it is difficult to visualize thethree-dimensional array of beams. To facilitate understanding, afunctional schematic diagram of a MEMS-based a 2-input, 2-output,7-wavelength wavelength cross connect 10 is illustrated in FIG. 1, asshown generally in the Smith et al. reference, although significantlygreater numbers of fibers and wavelengths are contemplated. Two inputfiber waveguides 12, 14 and two output fiber waveguides 16, 18 arealigned linearly parallel to each other to couple the waveguided signalsinto two free-space input beams 20, 22 and from two free-space outputbeams 24, 26. Thereafter, free-space optics guide and spatially switchthe light. By free-space optics is means optics based on free-standingand discrete optical elements, such as refractive or reflective lenses,mirrors, and diffraction gratings that optically process optical beamsthat are not waveguided but propagate in free space. As part of thefree-space optics, a lens 28 collimates the input beams 20, 22 to bothstrike a diffraction grating 30.

[0038] Considering the first input beam 20, the diffraction grating 30angularly disperses it into a fan-shaped collection 32 of beamsangularly separated according to wavelength. The wavelengths of thesignals on the one input fiber 12, as well as on all the other fibers12, 16, 18, correspond to the WDM wavelengths of one of the standardizedgrids, for example, the International Telecommunications Union (ITU)grid, and each optical carrier signal of the different wavelengths onthe separate fibers is modulated according to its own data signal. Eachof the beams in the beam collection 32 corresponds to one of thewavelength channels of the ITU grid. A lens 34 focuses these beamstoward a first row 76 of tiltable input mirrors 78, typically formed asa two-dimensional array 80 in the plane of a MEMS structure. The mirrors78 of the first row 76 are associated with the wavelength channels ofthe first input fiber 12 while those in a second row 82 are associatedwith the second input fiber 14. The mirrors 78 are also arranged in asecond dimension in which each column 84 is associated with one of thewavelengths λ₁ through λ₇ for the illustrated 7-wavelength system whereλ is the common symbol in the telecommunications industry for opticalwavelength. The mirrors 78 described to this point are input mirrors.Similarly arranged mirrors 86 in rows 88, 90 are output mirrors. Themirrors 78, 86 are tiltable about respective pairs of perpendicular axeslying generally horizontally in the illustration so that the inputmirrors 78 direct each input beam 20, 24 toward a folding mirror 92.Depending upon the tilt angle of the respective input mirror 78, thefolding mirror 92 reflects that beam to the output mirror 86 in aselected one of the output rows 88, 90. The selected output mirror 86 iscorrespondingly tilted to reflect the beam it receives from the foldingmirror 92 into a direction in the principal optical plane back towardsthe selected output fiber 16, 18. The two illustrated connections showcoupling of different wavelength components from the first input fiber12 to output mirrors 86 located alternatively in the third and fourthrows 88, 90. The output mirrors of the third row 88 are associated withrespective wavelength channels on the first output fiber 16 while thoseof the fourth row 90 are associated with the wavelength channels on thesecond output fiber 18. The optics are arranged and controlled such thatan optical signal from an input mirror 78 is reflected only to one ofthe output mirrors 86 in the same column 84, that is, to one associatedwith the same WDM wavelength. The input and output mirrors 78, 86typically have the same construction and differ only by their placementin the two-dimensional array 80 in a single MEMS structure. Practicallyspeaking, in this configuration, the designation of input and outputmirrors is arbitrary and the input and output rows may be interleaved.

[0039] However, there are other switch designs in which the input andoutput mirrors are formed in separate MEMS arrays. Such a dual arrayallows the elimination of the folding mirror. In some applications, itis possible to use only a single array of micromirrors without a foldingmirror to directly reflect a wavelength-separated input beam back to aselected output fiber. However, this configuration presents problemswith uniformity of coupling and greatly complicates the optics and MEMSdesign.

[0040] Each output mirror 86 is also tiltable in correspondence to thetilt angle of the input mirror 76 to which it is coupled through thefolding mirror 92, which operates as a symmetry plane of the system. Asa result, the same optics 28, 30, 34 used to focus and demultiplex thebeams from the input fibers 12, 14 are also used to multiplex thewavelength-separated output beams onto the two output fibers 16, 18.That is, the diffraction grating 30 acts both as a demultiplexer on theinput and as a multiplexer on the output.

[0041] By means of the illustrated optics and MEMS micromirror array 80,a wavelength channel on either of the input fibers 12, 14 can beswitched to the same wavelength channel on either of the output fibers16, 18. It is of course understood that the described structure may begeneralized to more input and output fibers and to more WDM wavelengths.

[0042] The following description of the structure of a cross connect ofthe invention will be simplified by the appreciation that there is nophysical distinction between input and output beams. The same opticsperform reciprocal functions, such as focusing and defocusing (e.g.,collimating), on beams traversing the optics in opposite directions.Therefore, the discussion will largely concentrate on the input beamsbut will apply as well to the output beams propagating in the reversedirection.

[0043] With this understanding of the functions of the system, a morepractical wavelength cross connect 100 illustrated in the orthographicview of FIG. 2 is designed to switch 40 wavelength channels on 100 GHzfrequency spacings (about 0.8 nm in wavelength at 1550 nm) between 6input fibers and 6 output fibers. That is, the exemplary design is basedon 12 fibers and 40 wavelengths. According to one nomenclature, thedesign results in a 12^(f)×40^(w) WXC. The required components of thisWXC can fit in a package within a form factor 102 of about 3.8×20×23 cm.In an overview, the cross connect 100 includes a concentrator 104,typically implemented in a single chip, which receives the twelve fibersin a linear alignment. Twelve waveguides having curved shapes within theconcentrator 104 are coupled to the twelve fibers and bring their beamscloser together on the side of the cross connect 100 and output thebeams in parallel in a linearly spaced grid. A collimation lens system106 collimates the beams diverging from the concentrator waveguides anddirects them in a linearly arrayed set of twelve beams to a diffractiongrating 108. The diffraction grating 108 spectrally separates the beamsinto twelve fan-shaped beams. Only a single fan-shaped beam isillustrated, and only the sub-beams associated with the lowest, middle,and highest wavelengths λ₁, λ₂₀, λ₄₀ of forty wavelengths are explicitlyshown although there are forty sub-beams in total for each fiber in thedescribed embodiment. Two planar mirrors 110, 112 reflect the beams andextend their path lengths before they pass through a lens 114 and aquarter-wave plate 116. The lens 114 focuses each of the two-dimensionalarray of beams at the symmetry point of the system to be describedlater. Two more planar mirrors 118, 120 reflect the beams and extendtheir paths lengths within the form factor 102 before they strikerespective tiltable micromirrors in a 12×40 MEMS chip 122. The freespace optics allow the beams to cross each other as illustrated withoutany interference. The MEMS chip 122 may be tilted at 45° with respect tothe plane of the optics previously described so that the beams arereflected generally downwardly to a planar fold mirror 124 extendingparallel to the previously described plane. The fold mirror 124 forms asymmetry plane, and the lenses 106, 114 focus the spectrally separatedoutputs of the fibers on the fold mirror 124.

[0044] The operation of the MEMS mirror array 122 typically limits theperformance of the system and hence drives the optical design.Therefore, MEMS technology and the arrangement of the mirror array willbe described first. There are several ways of forming the MEMSmicromirror array. An exemplary structure shown in plan view in FIG. 3and in cross section in FIG. 4 provides two-dimensional tilting of amirror 128 about two orthogonal axes 130, 132. The illustrated cell isone of many such cells arranged typically in a two-dimensional array ina bonded structure including multiple levels of silicon and oxide layersin what is referred to as a multi-level silicon-over-insulator (SOI)structure. The cell includes a gimbal structure of a frame 134 twistablysupported in a support structure 136 of the MEMS array through a firstpair of torsion beams 138 extending along and twisting about the minoraxis 130. The cell further includes a mirror plate 140 having the mirror128 formed thereon as a reflective surface 128. The mirror plate 140 istwistably supported on the frame 134 through a second pair of torsionbeams 142 arranged along the major axis 132 perpendicular to the minoraxis 130 and twisting thereabout. In one MEMS fabrication technique, theillustrated structure is integrally formed in an epitaxial (epi) layerof crystalline silicon. The process has been disclosed in U.S.Provisional Application Serial No. 60/362,898, filed Mar. 8, 2002,incorporated herein by reference in its entirety. However, otherfabrication processes resulting in somewhat different structures may beused without affecting the present invention.

[0045] The structure is controllably tilted in two independentdimensions by a pair of electrodes 144 under the mirror plate 140 andanother pair of electrodes 146 under the frame 134. The electrodes 144,146 are symmetrically disposed as pairs across the axes 130, 132 oftheir respective pairs of torsion beams 138, 142. A pair of voltagesignals V_(A), V_(B) are applied to the two mirror electrodes 144, andanother pair of voltage signals are applied to the frame electrodes 146while a common node voltage signal V_(c) is applied to both the mirrorplate 140 and the frame 134.

[0046] Horizontally extending air gaps 148, 150 are formed respectivelybetween the frame 134 and the support structure 136 excluding the onepair of torsion beams 138 and between the mirror plate 140 and the frame134 excluding the other pair of torsion beams 142 and overlie a cavityor vertical gap 152 formed beneath the frame 134 and mirror plate 140 sothat the two parts can rotate. The support structure 136, the frame 134,and the mirror plate 140 are driven by the common node voltage V_(c),and the frame 134 and mirror plate 140 form one set of plates forvariable gap capacitors. Although FIG. 3 illustrates the common nodevoltage V_(c) being connected to the mirror plate 140, in practice theelectrical contact is made in the support structure 136 and electricalleads are formed on top of the torsion beams 138, 142 to apply thecommon node voltage signal to both the frame 134 and the mirror plate140 which act as a top electrodes. The electrodes 144, 146 are formed atthe bottom of the cavity 152 so the cavity forms the gap of the fourcapacitors, two between the bottom electrodes 146 and the frame 134, andtwo between the bottom electrodes 144 and the mirror plate 140.

[0047] The torsion beams 138, 142 act as twist springs attempting torestore the frame 134 and the mirror plate 140 to neutral tilt positionstypically parallel to the principal plane of the chip. Any voltageapplied across opposed electrodes exerts a positive force acting toovercome the torsion beams 136, 140 and to close the variable gapbetween the top electrode and the more strongly biased bottom electrode.The force is approximately linearly proportional to the magnitude of theapplied voltage, but non-linearities exist for large deflections. If anAC drive signal is applied well above the resonant frequency of themechanical elements, the force is approximately linearly proportional tothe root mean square (RMS) value of the AC signal. In practice, theprecise voltages needed to achieve a particular tilt are experimentallydetermined.

[0048] Because the capacitors in the illustrated configuration arepaired across the respective torsion beams 138, 142 the amount of tiltis determined by the difference of the RMS voltages applied to the twocapacitors of the pair. The tilt can be controlled in either directiondepending upon the sign of the difference between the two RMS voltages.

[0049] As shown in FIG. 4, the device has a large lower substrate region154 and a thin upper MEMS region 156, separated by a thin insulatingoxide layer 158 but bonded together in a unitary structure. The tiltingactuators are etched into the upper region 156, each actuator suspendedover the cavity 152 by several tethers associated with the torsion beams138, 142. The electrodes are patterened onto the substrate 154, whichcan be an application specific integrated circuit (ASIC), a ceramicplate, a printed wiring board, or some other substrate with conductorspatterned on its surface. The actuators in the upper region form asingle electrical node called the “common node”. Each actuator issuspended above four electrodes 144, 146, each electrode being isolatedfrom every other electrode. To cause the actuator to tilt in a specificdirection, an electrostatic force is applied between the actuator andone or more of its electrodes by imposing a potential difference betweenthe common node and the desired electrode. Each actuator has two pairsof complementary electrodes, one pair 146 causing tilt about the majoraxis 132 and the other pair 146 causing tilt about the minor axis 130.Fabrication details are supplied in the aforementioned provisionalapplication 60/362,898.

[0050] The mirrors 128 of the mirror array are formed within a singlesubstrate 154 in a rectangular two-dimensional array arranged in a fiberdirection and a wavelength direction. Tilting of the mirrors 128 aboutthe respective major axes switches optical signals between the input andoutput fibers while tilting about the respective minor axes tunes theoptical coupling and may be used for intentional power degradation asdescribed by Smith et al. in the aforementioned patent application.Garverick et al. disclose an electronic method for controlling themirror tilt in U.S. patent application Ser. No. 09/884,676, filed Jun.19, 2001.

[0051] A typical mirror arrangement, as illustrated in the fragmentaryplan view of FIG. 5 includes mirrors 128 each having dimensions of 470μm in the wavelength direction and 750 μm in the fiber direction. Themirrors are arranged on spacings of 665 μm in the wavelength directionand 1050 μm in the fiber direction. The optics are designed to irradiateeach mirror 128 with an elliptically shaped spot 160, the edges showingthe e⁻² intensity contours relative to the center. The spot 160 has aminor axis in the wavelength direction of 290 μm and a major axis in thefiber direction of 400 μm. The ellipticity arises mainly from the 45°tilt of the MEMS chip 122. A 12×40 array 152 of such mirrors 128 isillustrated in the plan view of FIG. 6. The two-dimensional array 152spans about 25.9 mm in the wavelength direction and 11.7 mm in the fiberdirection.

[0052] The twist switching through a pair of mirrors in illustrated inthe schematic illustration of FIG. 7, which illustrates one wavelengthrow of twelve tiltable mirrors 128 linearly arranged in the MEMS chip122 along the fiber direction. Each mirror 128 is associated one of theinput and output fibers and is correspondingly either an input mirror oran output mirror. One constraint in designing the torque beam tilting isthe maximum tilt angle. The maximum tilt angle can be minimized bybracketing the input mirrors between the output mirrors or vice versa.For example, as illustrated in FIG. 7, the centermost mirrors, that is,the fourth through ninth mirrors 128, are input mirrors associated withrespective ones of the input fibers and arranged in a contiguous set166, and the outermost mirrors, that is, the first through third and thetenth through twelfth mirrors 128, are output mirrors associated withrespective ones of the output fibers bracketing the set 166 of inputmirrors 128. The effect is maximized when an equal number of outputmirrors are disposed on each side. For equivalent effect, the inputmirrors may be on the outside and the output mirrors on the inside.

[0053] To couple the fourth, input fiber to the tenth, output fiber asillustrated, the two associated mirrors 128 are twisted such that themirror reflected beams are specularly reflected from the fold mirror.That is, the two reflected beams are incident on the same spot of thefold mirror 124 at equal but opposite angles about a normal to themirror 124 at that spot. As illustrated in FIG. 7, beams F4 and F10associated with the fourth and tenth fibers respectively are tiltedslightly with respect to the principal plane of the optics, to which theillustrated dashed lines are parallel. The amount of beam tiling becomesincreasingly larger with increasing distance from the principal opticalplane, which by design intersects the middle of the MEMS mirror array122. In the figure, the central optical plane passes between the sixthand seventh mirrors 128. The beams willl tilt in a “toe-in” direction oneither side of the optical axis. That is, the beams tend towards theoptical axis as they approach the MEMS array 122. While this effect isnot a necessary condition for the design or function of the invention,it illustrates a significant design freedom in the optical systemwhereby the maximum required MEMS mirror tilt angle can be reduced bythe maximum amount of beam tilting occurring at the outer fibers.

[0054] The concentrator 104, as schematically illustrated in FIG. 8interfaces widely separated optical fibers 170 with the closely spacedparallel beams preferred for the free-space optics of the WXC 100 ofFIG. 2. The multiple fibers 170 are typically bundled in a planar ribbon172. Unillustrated V-shaped grooves in a holder substrate 176 hold thefibers 170 with a spacing of, for example, 127 μm. Although a core ofthe fiber 170 has a relatively small size of about 8 μm, its outer glasscladding results in a fiber diameter of approximately 125 μm. The largenumber of fibers which can be handled by the single set of free-spaceoptics of the invention arranged along an optical axis make it difficultto process a large number of fiber beams with such a large spacingbetween them because the outermost fiber beams are so far off theoptical axis. Also, as discussed in more detail below, a significantamount of optical magnification is required between these fibers and theMEMS mirror array, and the MEMS design and function are greatlysimplified as a result of concentrating the fiber spacing.

[0055] Instead, according to the invention, the fibers 170 held in theholder substrate 176 meet at a holder output face 182 with an input face184 of a waveguide concentrator chip 186, which includes a number ofsingle-mode, perhaps planar waveguides 188 formed in the concentratorchip 186. Typically, epoxy is used to join the fiber holder 176 and theconcentrator chip 186 and to index match the fibers 170 and the planarwaveguides 188.

[0056] The waveguides 188 at the input face 184 of the concentrator chip186 have a spacing matching that of the fibers 170 aligned to the fiberholder 176 but curve over a length of about 20 mm to create a reducedspacing at an output face 190 of the concentrator chip 186, for example,30 or 40 μm and preferably no more than 50 μm.

[0057] The planar waveguides 188 can be easily formed by a conventionalion exchange technique, such as is available from WaveSplitterTechnologies of Fremont, Calif. As illustrated in the cross-sectionalview of FIG. 9, a silicate glass substrate 190 is formed of a amorphoussilica (SiO₂) network through which sodium ions (Na⁺) are relativelymobile. The principal surface of the glass substrate 190 is covered witha metal mask 192 having a plurality of apertures 194 corresponding tothe waveguide pattern of FIG. 8. The patterned glass substrate 190 isplaced in a molten salt bath 196 of A⁻M⁺, where the anion A⁻ istypically NO₃ and the monovalent cation M⁺ may be Ag⁺. A very activeNa⁺/Ag⁺ ion exchange occurs in the substrate 190 creating doped regions198 beneath the mask apertures. The doped regions 198 have a higherrefractive index than the surrounding undoped glass and thus can serveas optical waveguides. However, their half-elliptical shape is opticallydisadvantageous. Therefore, after completion of ion exchange, a verticalelectric field is applied to the substrate to draw the positive ionsinto the glass substrate 190 to create nearly circular doped regions 200illustrated in the cross-sectional view of FIG. 10. These serve as theplanar optical waveguides 188 surrounded on all sides by the lower-indexglass. Other methods are available for forming planar waveguide.

[0058] The fibers 170 of FIG. 8 are aligned to the planar waveguides 188at the input face 184 of the concentrator chip 186. Preferably, thefiber end faces are inclined by about 8° to the planar waveguides 188 inorder to virtually eliminate back reflections between the fibers 170 andwaveguides 188.

[0059] Another type of concentrator 104 relies entirely upon opticalfiber. A fiber holder substrate 210, illustrated in the cross-sectionalview of FIG. 11 is patterned by precision photolithographic techniqueswith a series of V-shaped grooves 212 in the general planar pattern ofthe waveguides 188 of FIG. 8. Single-mode optical fibers 214 havingcores 216 surrounded by claddings 218 are butt coupled to theinput/output fibers 170 or are simply continuations of them. Typicalcore and cladding diameters are respectively 8.2 μm and 125 μm. However,for the illustrated concentrator, the claddings 218 at the ends of thefibers 218 are tapered to have a diameter corresponding to no more thanthe desired core-to-core spacing at the output of the concentrator, forexample, 40 μm, but with sufficient cladding thickness to providewaveguiding. The tapered fibers 214 are placed into the grooves 212 withtheir tapered ends forming inputs to the free-space cross connect. Theall-fiber design eliminates the tedious alignment and in-path epoxyjoint of the combination waveguides of FIG. 8. It also eliminatespolarization-related effects arising in planar waveguides.

[0060] The concentrator creates a relatively narrow spread of parallelfree-space beams in a linear arrangement for the wavelength crossconnect. Even when twenty-four fibers are connected to the crossconnect, they are concentrated to an overall width of only about 1 mm.This narrow beam spread substantially eases the off-axis performancerequired of the free-space optics and reduces the extend of the MEMSarray in the fiber direction. The design allows shorter focal lengthlenses and significantly reduces the overall size of the package. It isalso more reliable and highly tolerant to environmental stress. Withouta concentrator, the number of fibers connected to the cross connectwould be severely limited.

[0061] The collimating lens system 106 is illustrated in more detail inthe cross-sectional view of FIG. 12. The free-space beams output by thewaveguides, whether planar or fiber, of the concentrator 104 aredivergent and have a curved field. This discussion will describe all thebeams as if they are input beams, that is, output from the concentratorto the free-space optics. The beams are in fact optical fields coupledbetween optical elements. As a result, the very same principles apply tothose of the beams that are output beams which eventually reenter theconcentrator 104 for transmission onto the network.

[0062] The beam output from the concentrator 104 into the cross connectpass through a field-flattening lens 220 in order to flatten what wouldotherwise be a curved focal plane of the collimator lens. Thefield-flattening lens 220 accepts a flat focal plane for the multipleparallel beams emitted from the concentrator. In the reverse direction,the field-flattening lens 220 produces a flat focal plane and parallelbeams compatible with the end of the concentrator 104 to assure goodcoupling to the single-mode waveguide in the concentrator.

[0063] In many optical systems, an image is formed on a curved,non-planar surface, typically by beams non-parallel to each other. Inmany applications such as photographic imaging systems, such minordeviations from a flat field are mostly unnoticeable andinconsequential. However, for a cross connect based on free-spaceoptics, parallel single-mode fibers, small parallel beams, and planarmirror arrays, a curved image can degrade coupling efficiency atpositions displaced away from the optical axis. Performance is greatlyimproved if the optics produce a flat focal plane at the concentrator104 and at the fold mirror 124 since an image of the waveguides at theface of the concentrator exits at the fold mirror 124, and on the returntrip the fold mirror 124 will be imaged onto the concentrator waveguideends. Hence the ends of the input waveguides in the concentrator 104 areimaged onto the ends fo the output waveguides in the concentrator 104,and the efficiency of coupling into the single-mode waveguides stronglydepends on the quality of the image. Without the field-flattening lens,it would be very difficult to build a WXC with more than a few fiberports because the error in focus would significantly increase for fibersdisplaced away from the optical axis. A field-flattening lens isdesigned as an optical element with negative focal length and is thickerat its periphery than at its optical axis in the center. The basicfunction of the thicker glass at the periphery is to delay the focus ofthe beams passing therein. The delayed focus serves to create a flatplane of focus points for all beams rather than a curved locus of focithat would occur otherwise. A field-flattening lens may be implementedas a singlet lens, a doublet, or other lens configuration.

[0064] A field-flattening lens may in the absence of further constraintsproduce an optical field in which the off-axis beams approach the flatfocal plane at angles that increasingly deviate from normal away fromthe optical axis. Such non-perpendicular incidence degrades opticalcoupling to fibers arranged perpendicular to the flat focal plane.Therefore, performance can be further improved if the the beams are madeto approach the focal plane in parallel and in a direction normal to theflat focal plane. This effect of producing parallel beams is referred toas telecentricity, which is aided by long focal lengths.

[0065] After the field-flattening lens 220, the beams pass through acollimating doublet lens 222 consisting of a concave lens 224 joined toa convex lens 226. The doublet lens 222 may be a standard lens such asModel LAI-003, available from Melles Griot, which offers superiorcollimating and off-axis performance. The effective focal length f₁ ofthe assembly may be about 14 mm. The collimating lens 222 is illustratedas following the field-flattening lens 220, but their positions can bereversed with little change.

[0066] A prism 228, which may be a simple wedge, is placed between thecollimating lens 222 and the diffraction grating 108. The prism 228pre-corrects for the astigmatism introduced by the diffraction grating108. The wedge angle of the prism, along with the type of glass fromwhich it is made, allows elliptically shaped (or astigmatic) beams to becreated. If the prism 228 is composed of the common BK& optical glass,the wedgle angle is typically on the order of 25° to compensate for thetype of diffraction grating 108 considered for the invention. Theellipticity counteracts a similiar ellipticity that is an undersirableby-product of diffraction gratings. The net result of the prism andgrating is a distortion-free optical beam that can be efficientlyprocessed by the remaining optical components in the system andultimately coupled with high efficiency back into the small core of asingle-mode fiber. The field-flattening lens, collimating doublet lens22, and the prism 228 are hereafter referred to as the front-end optics.

[0067] The next major component is the diffraction grating 108 of FIG. 2in which the grating lines are perpendicular to the principal opticalplane of the wavelength cross connect 100. The diffraction gratingangularly separates the multi-wavelength input beams intowavelength-specific sub-beams propagating in different directionsparallel to the principal optical plane, or alternatively serves torecombine single-wavelength sub-beams into a multi-wavelength beam. Thegrating 108 is uniform in the fiber direction perpendicular to theprincipal optical plane, and its uniformity allows its use for signalsto and from multiple input and output fibers. The line density of thegrating should be as high as possible to increase spectral dispersionbut not so high as to severely reduce diffraction efficiency. Twoserially arranged gratings would double the spectral dispersion.However, a single grating with a line density of 1100 lines/mm hasprovided satisfactory performance. The grating is aligned so that thebeam from the collimating lens system 106 has an incident angle of 54°on the grating 108, and the diffracted angle is about 63°. Thedifference in these angles results in optical astigmatism which iscompensated by the previously mentioned prism 228. The potentiallysignificant polarization sensitivity of the grating is mitigated by thequarter-wave plate 116, to be described somewhat later. In brief, thediffraction efficiency of a grating is generally dependent on thecharacteristics of the polarization of the light with respect to thegroove direction on the grating, reaching extrema for linearpolarizations that are parallel and perpendicular to the grooves.

[0068] After the diffraction grating 108, the next major component isthe long-focus lens 114, which forms part of the back-end optics. It ispreferably a doublet lens having a focal length f₂ determining theoptical magnification in the fiber direction according to the ratiof₂/f₁. The magnification sets the inter-mirror spacing for a givenwaveguide spacing of the concentrator 106 since the the MEMS plane is ator very close to a conjugate image of the concentrator 106. An exampleof the long-focus lens 114 is Model LAO-277 available from Melles Griothaving a focal length f₂ of 355 mm. In combination with the previouslydescribed collimating lens, the inter-fiber magnification is mostpreferably about 25. A minimum of magnification of 10 and preferably ofat least 20 is desired so that the single-mode waveguide ends, which arespaced as closely together as possible to ease the optical design, willimage on the MEMS mirror array in beam widths many times larger than theoptical wavelengths. The magnification also allows easy fabrication ofreasonably sized MEMS mirrors. However, the magnification should not beso large, for example no larger than 50 or 100, that the MEMS chipbecomes too large for acceptable yields. As a result, a 30 μm waveguidespacing in the concentrator 106 requires a 1050 μm MEMS mirror spacingin the fiber direction when an additional factor of 1.414 is introducedto account for the 45° tilt of the MEMS mirror array.

[0069] The focal length f₂ of the long-focus lens 114 also determinesthe spot size of the optical beams on the MEMS mirrors and the depth offocus, alternately expressed as the Rayleigh length Z₀. A long Rayleighlength prevents excessive beam divergence in the space between the MEMSmirror and the folding mirror. However, Z₀ should be no longer thannecessary since the spot size on the mirror is proportional to Z₀, andsmaller spot sizes are generally preferred.

[0070] The long-focus lens 114 should be placed one focal length f₂ awayfrom the diffraction grating 108 for reasons related to those discussedpreviously for the telecentricity of the front-end optics. With a singlelens 114 in the back-end optics the function of telecentricity in thewavelength direction is achieved by simply placing the lens one focallength away from the grating 108. However, this invention does notdepend on the use of just one lens in the back-end optics. In fact, theoverall length of the back-end optical path can be physically compressedon the order of 50% to permit shrinking of the package by employingadditional optical elements in a traditional telephoto arrangement. Inthis case telecentricity is achieved by subtly changing the curvature oflens surfaces, and the specific design is greatly aided bycomputer-automated optimization algorithms such as CodeV produced byOptical Research Associates pf in Pasadena, Calif.

[0071] In the defined fiber direction of the optical systemtelecentricity is not a critical issue, and “toe-in” beams can even bebeneficial in the reducing the required major-axis tilt angle of theMEMS mirrors as discussed previously. However, in the wavelengthdirection of the optical system, if the beams approach the MEMS array122 at non-normal angles, then the MEMS mirrors must be able counteractthis deviation by tiliting about their wavelength (minor) axis. Inaddition, the position of the MEMS array 122 along the optical axisbecomes critical in order to properly intersect the mirrors with theangularly deviating beams of light, which adds to the alignmentcomplexity of the optical system. It is therefore desirable, especiallyin the wavelength direction, to produce beams of light that areparallel, or telecentric, to the optical axis of the system. Thetelecentricity assures that the sub-beams angularly diverging from thediffraction grating 108 approach the MEMS mirrors in parallel at leastwith respect to the wavelength direction. As a result, the position ofthe mirrors along the beams becomes less critical. The parallelism alsoprevents angular deviation on the return path. Any angular deviationbetween the wavelength-separated sub-beams would cause returning beamsto strike the diffraction grating 108 at incorrect angles and make italmost impossible to recombine the multiple wavelength signals into asingle output fiber. Also, the mirrors need not be steerable in theplane of dispersion, that is, about the minor axis, in order tocompensate for the angle of the beams. The elimination of steerabilityin the second direction allows micromirrors to be used having only asingle steering axis rather than two, thereby greatly simplifying theMEMS structure, its manufacturing, and the associated control circuitryso that the wavelength-separated sub-beams propagate in parallel. Ifonly one tilting axis is needed, specifically the major axis, the numberof electrodes and associated electrodes is reduced by one-half, and theresulting reduction in heat load benefits both reliability and lifetime.

[0072] For a WXC having significantly more fiber ports or wavelengthsthan the number illustrated in FIG. 6, an additional lens can be addedto the back-end optics to flatten the image field produced by them. Theadditional field flattening becomes necessary as the beam paths for theadditional fibers or wavelengths deviate farther from the central opticaxis such that the Rayleign length is no longer adequate by itself toabsorb the difference in path lengths occurring between the on-axis andoff-axis beams. Analogously to the field flattening in the front-endoptics, a back-end field-flattener lens allows the focus of all possiblebeams to form in a common flat plane. As discussed previously, theoptical system functions to image the flat face of concentrator 104 onthe fold mirror 124 and then back again onto the concentrator face in around trip through the switching optics. As a result of fieldflattening, the effect of path length differences between the variousbeams becomes inconsequential. Thus, for very large numbers of fibersand wavelengths, care must be exercised in the back-end optics toachieve the necessary level of both telecentricity and field-flattening.Otherwise, insertion loss becomes unacceptable for off-axis beams.

[0073] The quarter-wave plate 116 is placed between the diffractiongrating 108 and the MEMS array 122, preferably immediately after thelong-focus lens 114. Every wavelength-separated sub-beam passes twicethrough the quarter-wave plate 116 so that its polarization state isaltered between input and output. Therefore, the diffraction grating 108twice diffracts any wavelength-specific sub-beam, which has twice passedthrough the quarter-wave plate 116, once with a first (and arbitary)polarization and once again with a polarization state that iscomplementary to the first polarization state from the perspective ofthe diffraction grating 108. As a result, any polarization dependenceintroduced by the diffraction grating 108 is canceled. That is, the netefficiency of the grating 108 will be the product of its S-state andP-state polarization efficiencies and hence independent of the actualpolarization state of the input light.

[0074] The beams and sub-beams described to this point propagatesubstantially parallel to a horizontal principal optical plane. Anydeviation from the plane is less than 5° and preferably less than 1°,the least deviation being the best. However, the MEMS array 122 istilted at 45° to the principal optical plane and the folding mirror 124is positioned beneath it in a plane parallel to the principal opticalplane and perpendicular to the optical axis after reflection from themicromirrors. As a result, the sub-beams propagate substantiallyvertically between the MEMS 122 and the folding mirror. Such a bentarrangement conveniently separates the beam switching area from thepaths of the incoming and outoing beams but is not strictly depedent ona 45° MEMS tilt.

[0075] The switched optical connection of corresponding wavelengthchannels between input and output fibers proceeds as follows. An opticalsub-beam corresponding to a unique input fiber and optical wavelengthstrikes the corresponding input MEMS mirror and is deflected towards thefold mirror. The angle of the input mirror is actively controlled bydrive circuitry so that the sub-beam, after striking the fold mirror,lands precisely at the center of the desired output mirror. The outputmirror is actively tilted by the drive circuitry to the required anglesuch that the sub-beam, after reflection, is properly aligned to theconcentrator waveguide associated with the particularly output mirror.

[0076] In view of this procedure, an effective optical path, that is,switch state, can be created only when an input and an output mirror aremutually co-aligned. In the described design, the MEMS tilt accuracyshould be better than 0.01° to obtain the desired low insertion loss.The need to mutually align two micromirrors provides channel isolationbecause light from other fiber/wavelength combinations is significantlyblocked from entering the complex chosen path, thereby resulting inextremely low optical cross talk (alternatively phrased as highisolation or directivity). Low cross talk is typically critical for WDMtelecommunications systems. Further, light of a particular wavelengthcan be switched from any input fiber to any output fiber without concernfor any other optical paths in the system. The extremely low transientcross talk makes the free-space WXC a true non-blocking switch.

[0077] The described embodiment was based on a 40 channels in the 1550nm band. However, the design is easily adapted to conform to variousregions of the optical spectrum, including S-band, C-band, and L-band,and to comply with other wavelength grids, such as the the 100 GHz, 50GHz, etc. grids published by ITU.

[0078] The described design can be implemented with MEMS mirrors capableof tilting by ±4.6° in the major axis. The minor axis tilt for atwo-axis MEMS mirror array may be significantly less since it typicallyused for tuning, not switching. Such MEMS mirror arrays are possiblewith existing technology.

[0079] The described design provides several advantages for its easyinsertion into WDM systems of either a few wavelengths or for dense WDM(DWDM) systems having many wavelengths. It produces lower polarizationmode dispersion (PMD) and low chromatic dispersion. Low PMD naturallyfollows from the free-space optics. Low chromatic dispersion is achievedby assuring the lenses are achromatic. The resulting performance exceedspresent-day industry standards.

[0080] The MEMS micromirror array drives most of the design requirementsand system cost. The 12×40 array described before is relatively large inthe wavelength direction. Such size is possible, but 100% chip yield maybe questionable until manufacturing techniques are further perfected.Further, there is a desire to increase the number W of WDM wavelengthsto 80 and perhaps greater. At present, a 12×80 micromirror array ischallenging though not impossible to fabricate. Further, increasing thebeam array field of FIG. 6 to 50 mm in the wavelength direction presentssignificant problems with off-axis aberrations in the optics.

[0081] A first embodiment of implementing a 12^(f)×80^(w) WXC whileusing two 12×40 MEMS arrays is illustrated schematically in the planview of FIG. 13. Two 12×40 MEMS arrays 232 a, 232 b are bonded onto acommon substrate 230 in mosaic style. If the replication is in thewavelength direction, the locations of the micromirrors in the twoarrays 232 a, 232 b must be carefully aligned in the wavelengthdirection since a common diffraction grating is producing the wavelengthseparated components on the two arrays 232 a, 232 b. If the replicationis in the fiber direction, the alignment is less critical since themirror tilt angles can be adjusted to couple the mirrors on thedifferent arrays 232 a, 232 b through the fold mirror. Of course, themosaic can be extended to yet larger one- and two-dimensional arrays ofarray chips.

[0082] A second embodiment illustrated schematically in FIG. 14 isparticularly useful for increasing the number of fibers, for example, a24^(f)×40^(w) WXC using two 12×40 MEMS micromirror arrays. A two-sidedbaseplate 240 supports equivalent optics on both its top side 242 andits bottom side 244 separated by a median plane 246. The optics on thetwo sides 242, 244 are distinguished according to whether they areassociated with input or output. Twelve input fibers are coupled througha first concentrator 104 a and a first collimator lines system 106 a toa first diffraction grating 108. A first focusing lens 114 a focuses the12×40 sub-beams toward a first 12×40 MEMS array 232 a. On the bottomside 244, twelve output fibers are coupled to a second concentrator 104b. A second collimating lens system 106 b, a second diffraction grating108 b, a second focusing lens 114 b, a second quarter wave plate 116 b,and a second 12×40 MEMS array 232 a are located on the bottom side 244in symmetric locations about the median plane 246 with correspondingoptical elements on the top side 242.

[0083] The two 12×40 MEMS arrays 232 a, 232 b are optically coupledthrough an aperture 248 in the base plate 240. A folding mirror is notrequired. Instead, the median plane 246 represents the symmetry plane ofthe system. It is of course appreciated that the baseplate can extendvertically with replicated optics on its two lateral side or that allthese optical elements can be placed side by side and supported on oneside of a baseplate. However, the two-sided arrangement minimizes thebeam length between the two MEMS arrays 232 a, 232 b and more easilyallows the extended beams paths for the rest of the optics of the sortshown in FIG. 2.

[0084] A third embodiment illustrated schematically in FIG. 15 isparticularly useful for doubling the number of wavelengths without theneed for significant change in most of the optical components. Inparticular, a 12^(f)×80^(w) optical switch can rely on two 12×40 MEMSmicromirror arrays. Six input fibers and six output fibers 214 arecapable of carrying 80 WDM channels, which may be chosen, for example,to comply with industry-standard wavelength grids such as promulgated bythe International Telecommunications Union (ITU). The twelve fibers 214coupled through the concentrator 104 and the collimating lens system 106to the diffraction grating 108 which produces 80×12 collimated beams 250identified according to the twelve fibers and the eighty WDM wavelengthsλ₁ to λ₈₀. A short-focus lens receives the collimated beams 250 andproduces condensing beams 254 that are focused on two mirrors 256 a, 256b. The mirrors are disposed so that the first mirror 256 a receives thecondensing beams 254 for wavelengths λ₁ through λ₄₀ for all the fibers214 while the second mirror 256 b receives the condensing beams 254 forwavelength λ₄₁ through λ₈₀ for all the fibers 214. The two mirrors 254a, 256 b can be replaced by a single prism. The two beams 258 reflectedfrom the respective mirrors 256 a, 256 b are expanding beams. Long-focuslenses 260 a, 260 b in conjunction with unillustrated quarter waveplates refocus the beams through respective 12×40 MEMS micromirrorarrays 122 a, 122 b onto associated fold mirrors 124 a, 124 b. Thespatial segregation of wavelength channels into parallel MEMS arrays canbe carried further.

[0085] The scaling to larger size switches can also be accomplished on asystems level mostly outside of the optical cross connect. By way ofillustration, a 12^(f)×80^(w) WXC illustrated in FIG. 16 relies upon twoindependent 12^(f)×40^(w) WXCs having respective 12×40 MEMS micromirrorarrays. Each of twelve input and output fibers 270 are connected to thecommon port of a respective 3 dB splitter 272. The two split-out portsof each 3 dB splitter 272 are connected via connecting fibers 274 torespective ones of a first and a second 12^(f)×40^(w) wavelength crossconnect (WXC) 276 a, 276 b. The 3 dB splitters 272 are typicallyimplemented by fusing together two fibers in a gradual Y-junction, andthey may serve as either a splitter or combiner depending upon whetherthe signal is arriving from the left on the single input/output fiber270 or from the right on one of the two connecting fibers 274. Thissimple implementation of a splitter results in an inherent 6 dB loss inthe in a round trip, but there is typically a smaller addition loss inthe actual device depending upon its quality of fabrication.

[0086] All eighty wavelength components from the twelve fibers 270 arecoupled to the 12^(f)×40^(w) WXCs 276 a, 276 b, each of which includesits own 12×40 micromirror array. However, each of the 12^(f)×40^(w) WXCs276 a, 276 b are designed to couple only some of the wavelengths totheir micromirror array and to suppress the rest. For example, referringback to FIG. 2, one of the 12^(f)×40^(w) WXCs 276 a, 276 b could haveits 12×40 MEMS chip 122 positioned to intercept only the first fortywavelength channels λ₁ to λ₄₀ while the remaining wavelength channelsλ₄₁ to λ₈₀ fall outside of the MEMS chip 122. An unillustrated absorber,potentially as simple as flat black paint, is placed in the paths of thesuppressed channels to prevent extraneous radiation within the crossconnect package. The other 12^(f)×40^(w) WXC has a complementary designin which the differently located 12×40 MEMS chip 122 selectivelyswitches the wavelength channels λ₄₁ to λ₈₀ while the remainingwavelength channels λ₁ to λ₄₀ are suppressed.

[0087] In this design, a particular wavelength channel for all twelvefibers is switched within one of the two 12^(f)×40^(w) WXCs 276 a, 276b. There is no question of needing to transfer signals between the twoWXCs 276 a, 276 b. When the switched signals are directed back to theoutput fibers 270, there should be no question of the splitters 272combining interfering signals of a same wavelength because theunswitched signals have been selectively absorbed.

[0088] The complexity of the MEMS arrays can be further reduced in amanner similar to the mosaic patterning of FIG. 13. As illustrated inFIG. 17, functional 12×40 micromirror arrays 280 a, 280 b for therespective 12^(f)×40^(w) WXCs 276 a, 276 b are formed by four 12×10 MEMSarrays 282 bonded to separate areas of substrates 284 with absorbingblank spaces 286 formed between the 12×10 MEMS arrays 282 in thewavelength direction. The extent of the individual blank spaces 286 inthe wavelength direction is the same as that of the 12×10 MEMS arrays282, allowing for some edge effects. However, the blank spaces 286 ofthe first 12×40 micromirror array 280 a are aligned with the 12×10 MEMSarrays 282 of the second 12×40 micromirror array 280 b, and vice versa.The effect is that the first 12^(f)×40^(w) WXC 276 a switches thewavelength channels λ₁-λ₁₀, λ₂₁-λ₃₀, λ₄₁-λ₅₀, and λ₆₁-λ₇₀ while thesecond 12^(f)×40^(w) WXC 276 b switches the remaining forty wavelengthchannels.

[0089] The design of FIG. 16 can be generalized to multiple stages ofsplitting or combining signals. As illustrated schematically in FIG. 18,the intermediate fibers 274 bearing the split-out signals of the firststage of splitters 272 are connected to the common ports of a secondstage of splitters 290. In turn, second intermediate fibers 292connected to the split-ports ports of the second-stage splitters 290 areconnected to respective ones of four 12^(f)×20^(w) WXCs 294. This designsimplifies the MEMS arrays. However, it introduces an inherent 12 dBsplitting loss and complicates the fiber wiring.

[0090] As should be apparent from FIG. 18, four 12^(f)×40^(w) WXCs wouldallow the total wavelength capacity to be increased to 160 wavelengths.

[0091] The undesirable excess splitting loss resulting from themulti-stage approach generally illustrated in FIG. 18 can besubstantially reduced by replacing the simple power splitters withcommercially available wavelength interleavers, such as those availablefrom Oplink Communications, Inc. and from WaveSplitter Technologies,inc. Interleavers function to split signal on the basis of wavelengthrather than power in that they send alternate wavelength out on separatefiber ports. For example, the even numbered wavelengths are segrated toexit the interleaver on one fiber while the odd numbered wavelengthsexit on another fiber. The insertion loss for an interleaver can be madeas low as about 1dB. Interleavers can be daisy-chained to provide thedesired degree of wavelength segregration. For example, in a two-stageinterleaver, every fourth wavelength exits on one fiber, etc. Viewed inthe reverse direction, interleavers operate to combine wavelengthcomponents. With reference again to FIG. 18, if the power splitters 272,290 are replaced one-for-one by interleavers, then each of the four12^(f)×40^(w) WXCs now addresses every fourth wavelength. Further, thedesign of each WXC is substantially relaxed by needing to operate on awavelength grid that is four times coarser than the original grid.Further, there in no longer a need for the channel suppression(absorber) regions previously described. Yet further, the inherentexcess splitting loss is reduced to about 4 dB compared to about 12 dBfor systems based on power splitters.

[0092] Both optical power splitters and interleavers are passivereciprocal devices, and both are included in a larger class of opticaldivider/combiners.

[0093] The wavelength cross connect illustrated in FIG. 2, even whenmodified according to some of the secondary embodiments, offers manyadvantages. However, the invention is not so limited and many aspects ofthe invention may be applied to other geometries and optical switchingdevices. A system, such as that of FIG. 14, that lacks the foldingmirror but includes separate input and output MEMS mirror arraysdirectly coupled together enjoys much the same capability as the systemof FIG. 2. Such a system may have combined optics including, forexample, the concentrator and diffraction grating, or may have separateinput and output optics. White-light systems switch the entiretransmission of one input fiber to a selected output fiber. Awhite-light cross connect can be adapted from the system of FIG. 2 byeliminating the diffraction grating. If the MEMS mirrors are tiltable intwo-dimensions, the fibers can be bundled in a two-dimensional array.

[0094] Other types of MEMS mirror arrays may be used, including thoserelying on flexing elements other than axial torsion beams and thosemoving in directions other than tilting about a central support axis.Wavelength dispersive elements other than diffraction gratings may beused. The concentrator, although important, is not crucial to many ofthe aspects of the invention. Further, the concentrator may beimplemented in an optical chip serving other functions such asamplification or wavelength conversion.

[0095] Although the invention has been described with respect to awavelength cross connect, many of the inventive optics can be applied towhite-light optical cross connects that do not include a wavelengthdispersive elements. Although tilting micromirrors are particularlyadvantageous for the invention, there are other types of MEMS mirrorsthan can be electrically actuated to different positions or orientationsto effect the beam switching of the invention.

[0096] The invention provides a small, rugged, and economical opticalcross connect capable of switching many wavelength channels between manyfibers. However, the many aspects of the invention may be applied toother applications than the described wavelength cross connect.

1. A wavelength cross connect, comprising: at least three opticalwaveguides having waveguide ends arranged in a first linear arrayextending in a first direction perpendicular to a principal opticalplane; a wavelength dispersive element uniform in said first directionand coupled to beams associated with each of said waveguide ends andspectrally separating wavelength components of said beams withwavelength-separated sub-beams disposed in a two-dimensional arrayarranged in a first waveguide direction and a first wavelengthdirection; a first set of free-space optics arranged along an opticalaxis extending in said principal optical plane and coupling saidwaveguides and said wavelength dispersive element; a plurality of microelectromechanical system (MEMS) mirrors arranged in a first mirror arrayarranged in at least a second wavelength direction and coupling at leastone of said waveguides to a selected one of others of said waveguides.2. The cross connect of claim 1, wherein said first mirror array is atwo-dimensional array additionally extending in a waveguide direction.3. The cross connect of claim 2, further comprising a fold mirrorcoupling pairs of mirrors in said first mirror array.
 4. The crossconnect of claim 1, further comprising a second mirror array of MEMSmirror extending at least in a third waveguide direction.
 5. The crossconnect of claim 1, further comprising a second set of free space opticsextending along said optical axis in said principal optical plane andcoupling said wavelength dispersive element to said first MEMS array. 6.The cross connect of claim 5, wherein beams passing between two opticalelements of said first and second set of free space optics cross beamspassing between two other optical elements of said first and second setof free space optics.
 7. The cross connect of claim 5, wherein amagnification of a combination of said first and second free spaceoptics is between 10 and
 100. 8. The cross connect of claim 1, whereinsaid wavelength dispersive element is a diffraction grating and whereinsaid first set of free space optics includes a prism compensating anastigmatism of said diffraction grating.
 9. The cross connect of claim1, further comprising a concentrator in which said waveguides are formedin a curved pattern spaced more closely together at said waveguide endsthan at portions away from said waveguide ends.
 10. A wavelength crossconnect, comprising: at least three optical waveguides having waveguideends arranged in a first linear array; a wavelength dispersive elementcoupled to beams associated with each of said waveguide ends andspectrally separating wavelength components of said beams withwavelength-separated sub-beams disposed in a two-dimensional arrayarranged in a first waveguide direction and a first wavelengthdirection; a first set of free-space optics coupling said waveguides andsaid wavelength dispersive element; a fold mirror; a plurality of microelectromechanical system (MEMS) mirrors arranged in a two-dimensionalarray arranged in a second waveguide direction and a second wavelengthdirection, pairs of said mirrors disposed at one location along saidsecond wavelength direction being movable to couple light through saidfold mirrors between respective ones of said pairs; and a second set offree-space optics coupling said wavelength dispersive element to saidMEMS mirrors and said fold mirror; wherein one of said waveguides isselectively coupled to either of two others of said waveguides.
 11. Thecross connect of claim 10, wherein said at least three waveguidescomprise at least two first waveguides and at least two secondwaveguides and wherein each of said first waveguides is selectivelycoupled to any of said second waveguides.
 12. The cross connect of claim11, wherein all of said first waveguides are arranged in said lineararray inside respective ones of said second waveguides.
 13. The crossconnect of claim 10, wherein said MEMS mirrors are formed in a plane.14. The cross connect of claim 13, wherein said MEMS mirrors are formedin a single substrate.
 15. The cross connect of claim 13, wherein saidbeams and sub-beams propagate substantially parallel to a principaloptical plane, and wherein said plane of said MEMS mirrors is inclinedwith respect to said principal optical plane.
 16. The cross connect ofclaim 15, wherein said fold mirror has a reflective surface parallel tosaid principal optical plane.
 17. The cross connect of claim 10, whereinsaid MEMS mirrors are tiltable about respective first axes to effectselective coupling through said fold mirror.
 18. The cross connect ofclaim 17, wherein said MEMS mirrors are titable about respective secondaxes perpendicular to respective ones of said first axes.
 19. The crossconnect of claim 10, wherein said waveguides are coupled to free-spacebeams propagatnig in parallel.
 20. The cross connect of claim 10,wherein said first set of free-space optics are arranged along anoptical path extending in a principal optical plane and wherein saidfirst linear array extends in a direction perpendicular to saidprincipal optical plane.
 21. A optical cross connect, comprising: aconcentrator having associated therewith at least three waveguideshaving ends arranged in a first linear array separated by a firstspacing and being separated from each other by a second spacing largerthan said first spacing away from said ends; a free-space optical systemcoupled to all of said ends; and a microelectromechanical systemincluding an array of movable mirrors coupled to all of said endsthrough said free-space optical system.
 22. The cross connect of claim21, wherein said first spacing is no more than 50 μm.
 23. The crossconnect of claim 21, further comprising a substrate in which saidwaveguides are formed as planar waveguides in a curved pattern, saidends being formed on a first side of said substrate and a second side ofsaid substrate being configured to couple said waveguides to respectiveones of at least three optical fibers.
 24. The cross connect of claim21, wherein said free-space optical system is arranged along an opticalaxis extending in a principal optical plane and wherein said firstlinear array is arranged along a direction perpendicular to saidprincipal optical plane.
 25. The cross connect of claim 21, wherein saidwaveguides are optical fibers and further comprising a substrate formedwith grooves formed in a curved pattern to align respective ones of saidoptical fibers.
 26. The cross connect of claim 25, wherein said opticalfibers are tapered toward said ends.
 27. An optical cross connect,comprising: an array of optical waveguides having ends and coupled tofree-space beams propagating substantially parallel to a principaloptical plane; free-space optics disposed in an optical path parallel tosaid principal optically interacting with said free-space beams; a microelectromechanical system (MEMS) array of movable mirrors disposed in aplane inclined to principal optical plane; and a fold mirrorreflectively coupling respective pairs of said mirrors.
 28. The crossconnect of claim 27, wherein said plane of said MEMS array is inclinedat 45° with respect to said principal optical plane and said fold mirrorhas a reflective surface parallel to said principal optical plane. 29.The cross connect of claim 27, wherein said free-space optics includes adiffraction grating and wherein said array of movable mirrors is atwo-dimensional array arranged in a waveguide direction and in awavelength direction.
 30. An optical cross connect, comprising: aplurality of optical waveguides arranged in an array and extending alongparallel paths to waveguide ends thereof: a collimating lens systemoptically coupled to said waveguide ends and including afield-flattening element; and an array of micro electromechnical system(MEMS) mirrors optically coupled through said collimating lens system tosaid waveguide ends.
 31. The optical cross connect of claim 30, furthercomprising: a wavelength dispersive element coupled to said waveguideends through said collimating lens system; and a focusing lensinterposed between said wavelength dispersive element and said array ofMEMS mirrors and having a focus point on said array of MEMS mirrors. 32.The optical cross connect of claim 30, further comprising a fold mirrorreflectively coupling pairs of said MEMS mirrors.
 33. The optical crossconnect of claim 30, further comprising: a wavelength dispersive elementcoupled to said waveguide ends through said collimating lens system; anda focusing lens interposed between said wavelength dispersive elementand said array of MEMS mirrors and having a focus point on an opticalpath between said array of MEMS mirrors and said fold mirror.
 34. Theoptical cross connect of claim 30, further comprising: a wavelengthdispersive element coupled to said waveguide ends through saidcollimating lens system; and a focusing lens interposed between saidwavelength dispersive element and said array of MEMS mirrors; wherein aratio between a focal length of said focusing lens and a focal length ofsaid collimating lens system is between 10 and
 100. 35. The opticalcross connect of claim 34, wherein said ratio is between 20 and
 50. 36.An optical cross connect coupled to a plurality of optical waveguidescarrying a plurality of wavelength multiplexed optical signals,comprising: at least one stage of a plurality of opticaldivider/combiners each coupled on one side to a one of said opticalwaveguides and including on another side a plurality of ports; and aplurality of wavelength cross connects each coupled to a respective oneof said ports of each of said optical divider/combiners and configuredto optically switch corresponding and different groups of wavelengthcomponents of said plurality of wavelength multiplexed optical signals.37. The cross connect of claim 36, wherein each of said wavelength crossconnects includes an array of micro electromechanical system (MEMS)mirrors and free-space optics including a wavelength dispersive element.38. The cross connect of claim 36, wherein said divider/combinerscomprises power combiner/dividers.
 39. The cross connect of claim 36,wherein said divider/combiners comprise interleavers.
 40. A method ofwavelength switching multi-wavelength signals conveyed on a plurality offibers, comprising the steps of: coupling light between said fibers andmulti-wavelength free-space beams propagating in parallel in a lineararray arranged parallel to a principal optical plane with spacings of nomore than 50 μm; spectrally coupling said multi-wavelength free-spacebeams and wavelength-separated free-space sub-beams; positioning atwo-dimensional array of tiltable mirrors to intercept respective onesof said sub-beams; and selectively tilting said mirrors to selectivelyoptically couple pairs of said fibers.
 41. The method of claim 40,wherein said two-dimensional array of tiltable mirrors is formed in acommon substrate and wherein said tilting includes electricallyactuating said mirrors.
 42. The method of claim 40, wherein said mirrorsare tiltable about two respective orthogonal axes.